Abnormalities in the human eye can lead to vision impairment such as myopia (near-sightedness), hyperopia (farsightedness), astigmatism, and presbyopia. A variety of devices and procedures have been developed to attempt to address these abnormalities.
One type of device that has been proposed is a corneal implant, such as an onlay, which is placed on top of the cornea such that the outer layer of the cornea (i.e., the epithelium), can grow over and encompass the onlay. An inlay is a corneal implant that is surgically implanted within the cornea beneath a portion of corneal tissue by, for example, cutting a flap in the cornea and positioning the inlay beneath the flap. An inlay can also be positioned within a pocket formed in the cornea.
Inlays can alter the refractive power of the cornea by changing the shape of the anterior surface of the cornea, by creating an optical interface between the cornea and an implant by having an index of refraction different than that of the cornea (i.e., has intrinsic power), or both. The cornea is the strongest refracting optical element in the eye, and altering the shape of the anterior surface of the cornea can therefore be a particularly useful method for correcting vision impairments caused by refractive errors.
LASIK (laser-assisted in situ keratomileusis) is a type of refractive laser eye surgery in which a laser is used to remodel a portion of the cornea after lifting a previous cut corneal flap.
Presbyopia is generally characterized by a decrease in the eye's ability to increase its power to focus on nearby objects due to, for example, a loss of elasticity in the crystalline lens that occurs over time. Ophthalmic devices and/or procedures (e.g., contact lenses, intraocular lenses, LASIK, inlays) can be used to address presbyopia using three common approaches. With a monovision prescription, the diopter power of one eye is adjusted to focus distant objects and the power of the second eye is adjusted to focus near objects. The appropriate eye is used to clearly view the object of interest. In the next two approaches, multifocal or bifocal optics are used to simultaneously, in one eye, provide powers to focus both distant and near objects. One common multifocal design includes a central zone of higher diopter power to focus near objects, surrounded by a peripheral zone of the desired lower power to focus distant objects. In a modified monovision prescription, the diopter power of one eye is adjusted to focus distance objects, and in the second eye a multifocal optical design is induced by the intracorneal inlay. The subject therefore has the necessary diopter power from both eyes to view distant objects, while the near power zone of the multifocal eye provides the necessary power for viewing near objects. In a bilateral multifocal prescription, the multifocal optical design is induced in both eyes. Both eyes therefore contribute to both distance and near vision.
Regardless of the vision correction procedure and/or devices implanted, it is important to understand the cornea's natural response to the procedure to understand how the cornea will attempt to reduce or minimize the impact of the vision correction procedure.
Specific to understanding a response to an inlay, Watsky et al. proposed a simple biomechanical response in Investigative Ophthalmology and Visual Science, vol. 26, pp. 240-243 (1985). In this biomechanical model (“Watsky model”), the anterior corneal surface radius of curvature is assumed to be equal to the thickness of the lamellar corneal material (i.e., flap) between the anterior corneal surface and the anterior surface of a corneal inlay plus the radius of curvature of the anterior surface of the inlay.
Reviews of clinical outcomes for implanted inlays or methods for design generally discuss relatively thick inlays (e.g., greater than 200 microns thick) for which the above simple biomechanical response model has some validity. This is because the physical size of the inlay dominates the biomechanical response of the cornea and dictates the primary anterior surface change. When an inlay is relatively small and thin, however, the material properties of the cornea contribute significantly to the resulting change in the anterior corneal surface. Petroll et al. reported that implantation of inlays induced a thinning of the central corneal epithelium overlying the inlay. “Confocal assessment of the corneal response to intracorneal lens insertion and laser in situ keratomileusis with flap creation using IntraLase,” J. Cataract Refract. Surg., vol. 32, pp 1119-1128 (July 2006).
Huang et al. reported central epithelial thickening after myopic ablation procedures and peripheral epithelial thickening and central epithelial thinning after hyperopic ablation procedures. “Mathematical Model of Corneal Surface Smoothing After Laser Refractive Surgery,” America Journal of Ophthalmology, March 2003, pp 267-278. The theory in Huang does not address correcting for presbyopia, nor does it accurately predict changes to the anterior surface which create a center near portion of the cornea for near vision while allowing distance vision in an area of the cornea peripheral to the center near portion. Additionally, Huang reports on removing cornea tissue by ablation as opposed to adding material to the cornea, such as an intracorneal inlay.
What is needed is an understanding of the cornea's response to the correction of presbyopia. An understanding of the corneal response allows the response to be compensated for when performing the procedure on the cornea and/or implanting an implant within the cornea, either of which can be used to alter the curvature of the cornea. A need also exists for understanding the cornea's response to a vision correction procedure that creates a center zone for near vision while providing distance vision in a region peripheral to the central zone.